An optical fiber is produced by drawing a preform on a drawing tower. A preform generally includes a primary preform consisting of a very high quality glass tube that forms a part of the cladding and the fiber core. This primary preform is then overcladded (or sleeved) to increase its diameter and form a preform that can be used on a drawing tower.
Homothetic fiber drawing includes placing the preform vertically in a drawing tower and drawing a strand of fiber from one end of the preform. For this purpose, one tip of the preform is heated locally until the silica softens. The drawing rate and the temperature are controlled during the drawing operation to determine the diameter of the resulting optical fiber.
The primary preform may be fabricated by successively depositing layers of pure and/or doped silica on the inside surface of a glass tube to form an inner cladding and a central core. The deposits in the glass tube are generally formed by chemical vapor deposition (CVD). This kind of deposition is conducted by injecting gaseous mixtures into the glass tube and ionizing the gaseous mixtures. CVD-type deposition encompasses MCVD (modified chemical vapor deposition), FCVD (furnace chemical vapor deposition), and PCVD (plasma enhanced chemical vapor deposition). The glass tube is then closed using a collapsing operation to form the primary preform.
The primary preform is then overcladded using silica particles, typically inexpensive natural silica particles to yield a final preform. The overcladding of the primary preform may be conducted by plasma deposition during which particles of natural silica are projected and fused by a plasma torch at a temperature near 2300° C. At such an elevated temperature, the natural silica particles vitrify on the periphery of the primary preform. The overcladding operation is generally conducted in a closed chamber under a controlled atmosphere to ensure protection against electromagnetic disturbances and the release of ozone that is emitted by the plasma torch.
The geometry of the preform must be well adapted to the ratios of the refractive indexes and diameters of the core and fiber cladding so that the drawn fiber has the required refractive index profile. For optical fibers, the refractive index profile is generally qualified in relation to a plotted graph showing the function associating the refractive index of the fiber with the radius of the fiber.
Precise control of the overcladding operation is therefore required to guarantee the homothetics of the geometry of the final preform and thus the drawn fiber. A target diameter of the final preform is calculated from the diameter of the primary preform and the target profile of the fiber. This target diameter determines the overclad quantity to be deposited on the primary preform. The overclad deposition is then conducted in one or more passes, wherein each pass corresponds to a translation movement of the plasma torch along the primary preform and to the deposition of a controlled quantity of silica particles. Hence, with every overcladding pass, a certain thickness of silica cladding is deposited on the primary preform.
In general, each overcladding pass provides a given thickness (e.g., four millimeters or so) corresponding to a given translation speed and a given silica particle flow rate. It is desirable to attain an overcladded preform that approaches its target diameter, such as accuracy in the order of 0.3 millimeter for a preform having a diameter of 90 to 100 millimeters. One problem, therefore, is to achieve the target diameter of the preform precisely when the second-to-last overcladding pass (i.e., the penultimate overcladding pass) brings the preform to an interim diameter in which less than one pass thickness is needed to complete the overcladding.
One solution is to reduce the flow rate of the silica particles projected onto the preform during the last overcladding pass in order to deposit a thinner overcladding layer and thereby reach the target diameter of the preform. This solution is described, for example, in European Patent Publication No. EP 1279646 A1 (and its counterpart U.S. Publication No. 2003/0024273 A1, which is hereby incorporated by reference in its entirety).
Equipment for overcladding an optical fiber preform is also known from European Patent Publication No. EP 0845441 A1 (and its counterpart U.S. Pat. No. 5,958,102, which is hereby incorporated by reference in its entirety), which describes a first depositing overclad torch along the preform and a plurality of second torches adapted so that each conducts a short back-and-forth movement over a given length of the preform. The longitudinal uniformity of the overcladding deposited along the primary preform is controlled, and one or more of the second torches may be actuated to correct any non-uniformity detected in the overcladding over a given length. The overcladding deposition by the first torch is conducted at a constant speed and with a variable particle flow rate to achieve the target diameter of the preform.
Reducing the flow rate of the silica particles projected at the end of the overcladding operation, however, has the disadvantage of reducing the yield of the overcladding process and inducing a productivity loss. The efficacy of silica particle projection increases with the diameter of the preform being overcladded. At the start of the overcladding operation, a large quantity of silica particles pass beside the primary preform because its diameter is small. Then, as the diameter of the preform increases with the overcladding, an increasing quantity of projected silica particles reaches the preform to be vitrified. In terms of yield with respect to the quantity of particles used, the efficacy of the overcladding operation increases with the diameter of the preform. Reducing the flow rate of the particles during the last pass thus cancels out this increase in yield and reduces the overall productivity of the overcladding operation of the preform.
Equipment for overcladding an optical fiber preform is also known from European Patent Publication No. EP 0482348 A2 (and its counterpart U.S. Pat. No. 5,183,490, which is hereby incorporated by reference in its entirety), which discloses continuously controlling the quantity of overcladding deposited using a calculation of the weight of the preform. When the preform reaches a target weight, the overcladding is stopped. This document, however, does not indicate how the end of the overcladding operation is controlled.
Therefore, there is a need for an easily implemented optical fiber overcladding method that facilitates the precise achievement of the target preform diameter without productivity loss.